1. Technical Field
This invention relates to apparatus and methods for purifying automotive engine emissions, and more particularly to such apparatus and methods which concurrently use catalysis and air/fuel ratio feedback to maximize purification.
2. Description of the Prior Art
To improve the conversion efficiency of a catalyst system, the prior art has followed essentially two paths: (i) change the chemical or physical arrangement of catalyst elements within the conversion chamber, or (ii) modify the gaseous emissions delivered to the catalyst conversion chamber. With respect to changing the catalyst elements, the prior art has commercially evolved a three-way catalyst (that which converts HC, CO and NO.sub.x) by use of a combination of precious metals coated onto a stabilized alumina substrate which in turn is carried on a monolithic ceramic cellular core. The core is engineered to pass emissions therethrough along a straight, uninterrupted axial flow at various space velocities. Such catalyst construction cannot, by itself, overcome conversion deficiencies due to random and statistically unpredictable oxygen level deviations (chemical noise) as well as statistically predictable deviations of the oxygen levels (transient excursions) which may occur, particularly during warm-up and cold-start of the engine. Chemical noise in the exhaust gas is due to mixing of turbulent rich and lean eddy currents that result respectively from variable cylinder exhaust gases and incomplete cylinder burns. Combustion explosions inherently vary from cylinder to cylinder due to different burn rates and different pulsing waves. The variation is compounded by the different lengths of exhaust ports leading to the mixing of the exhaust gases into a common stream. Chemical noise deviations cannot be predicted and do inhibit ideal oxygen level control. Transient excursions denote a temporary real change from an average oxygen level due to inadequate combustion control and should be accurately sensed for compensation. Transient excursions are not considered random and are predictable.
With respect to modifying the delivered emission gas, the prior art has found that the closer the combustion air/fuel (A/F) ratio in the engine is to stoichiometry, the lower will be the volume of noxious elements (HC, CO, NO.sub.x) delivered to the catalyst chamber for conversion. Although A/F ratio feedback systems have been deployed, transient and random combustion effects still prevail preventing the catalysts from achieving advanced levels of conversion efficiencies so necessary to anticipated mandated federal levels. Even small A/F measurement inaccuracies or small time delays in sensor detection and feedback result in hunting of the A/F adjustment and inability to achieve anticipated mandated federal levels for 1994 (gm/mi at between 75,000-100,000 miles of vehicle use) of 0.25-31 HC, 3.4-4.2 CO, 0.4 NO.sub.x, and 0.29-0.36 total hydrocarbons (THC). Moreover, such random effects are progressively exaggerated by aging of the catalyst and feedback system with time preventing the system from achieving anticipated federal long-life requirements beginning in years 2002 and beyond (at 100,000 miles) which may be essentially reduced to half of the anticipated 1994 requirements.
This invention has found dramatic improvement by uniquely combining a low mass, highly loaded filter catalyst upstream of a linear, wide-range, universal exhaust gas oxygen (UEGO) sensor in a feedback control loop with an A/F ratio modifier; such combination treats the exhaust gas before it enters a main conversion catalyst. Applicant is unaware of any prior art that (a) contemplates use of filter catalysts (those capable of filtering out random combustion effects or chemical noise) while converting usually only a minor amount of the noxious emissions from a moving vehicle; (b) uses a single UEGO sensor upstream of the main catalyst and downstream of the filtering catalyst; and (c) deploys automatic compensation for degradation of either the catalyst or sensor with time.
The first design of a basic nonswitching exhaust gas sensor, necessary to this invention, first appeared about 1981 with the issuance of the Hetrick U.S. Pat. No. 4,272,329 (assigned to Ford Motor Company). This patent describes a multiple-cell oxygen sensor that is more useful for meeting tighter emission standards. The sensor of this patent provides a linearized output that measures more accurately a wider range of A/F ratio while being substantially temperature insensitive to avoid thermally induced inaccuracies. Papers published during 1986 and 1988 show the acceptance in the technical community of such UEGO sensor when applied to an extended range of A/F ratios using a closed loop feedback system (closed loop being used herein to mean a controlled quantity as measured and compared to a standard representing desired performance). Such articles include: (1) I. Murase, A. Moriyama, and M. Nakai, "A Portable Fast Response Air-Fuel Ratio Meter Using An Extended Range Oxygen Sensor", SAE Paper 880559, Feb. 29, 1988; (2) J. Ishii, M. Amano, T. Yamauchi, and N. Kurihara, "Wide-Range Air-Fuel Ratio Control System", SAE Paper 880134, Feb. 29, 1988; (3) S. Ueno, N. Ichikawa, S. Suzuki and K. Terakado, "Wide-Range Air-Fuel Ratio Sensor, SAE Paper 860409, 1986; and (4) S. Suzuki, T. Sasayama, M. Miki, M. Ohsuga and S. Tanaka, "Air-Fuel Ratio Sensor For Rich, Stoichiometric and Lean Ranges", SAE Paper 860408, 1986.
What the design evolution of such UEGO sensor lacks is how to use it in a system to realize its accuracy potential. Applications of more primitive exhaust gas sensors (switching sensors) in the prior art have used (a) a single switching sensor with one or two catalyst bodies, or (b) dual switching sensors with one or two catalyst bodies. Switching sensors have a very steep change in signal at or about stoichiometry.
A single switch-type exhaust gas sensor in an upstream position relative to the catalyst body was used in U.S. Pat. No. 4,000,614 (1977). The sensor was placed in a feedback control loop and was only able to achieve A/F ratio control accuracies within .+-.1-2% of stoichiometry. This poor range of tolerance for A/F ratio is characteristic of excessive hunting and overcorrecting by the oxygen sensor due in part to its placement and in part to the employment of a switchpoint type sensor, characteristic of the sensors used during the 1970's.
An attempt was made in U.S. Pat. No. 3,961,477 (1976) to place the exhaust gas sensor between two catalyst bodies, the upstream body being an oxidation catalyst and the downstream body being a reducing catalyst. This patent is also an early example of closed loop A/F ratio control for catalysis and is representative of one of the most effective concepts of the 1970's. The A/F ratio tolerance capability is poor due in part to the use of a switchpoint-type exhaust gas sensor and the use of air injection immediately upstream of the sensor which detracts from its ability to accurately sense the oxygen content of the emissions. This reference does not describe the catalyst with respect to loading or effectiveness.
Use of more than one exhaust gas sensor is included in U.S. Pat. Nos. 3,939,654; 4,251,990; and 4,761,950. In the '654 patent (1976), step function (switch-type) oxygen sensors were placed upstream and downstream of a catalyst body while using a closed loop feedback system to a fuel injector for the engine. This patent properly cites problems with response time and accuracy for the oxygen sensors and attributes some of the problems to the catalyst itself, regardless of the type used. A comparator and an integrator were used to obtain control of the A/F ratio feedback control loop. Although long-term accuracy is increased somewhat from a single switchpoint sensor, accuracy was not improved much below .+-.1%.
In patent '990 (1981), dual sensors are used; an exhaust gas sensor is placed upstream of two catalysts in series. Again, switchpoint exhaust gas sensors are used accompanied by transient A/F ratio control inaccuracies and by time delay of compared signals leading to continued hunting and poor response.
The '950 patent (1988), employs software (see jumpback control algorithm in FIG. 4D) which uses downstream sensor information to modify an upstream sensor placed about a single catalytic body and then compares the information for control Purposes. Again, the sensors are of the switchpoint-type and the system suffers from A/F ratio control accuracy problems and comparator delay.
None of the above patented prior art improves the cycle response time of the sensors within a system or independently thereof.